To nitpick, we never learned whether the Edo had warp drive or not - we only learned they consider transporters somewhat miraculous (indeed, their God probably operates similar technology, and Deeds of God = Miracles, even if they are more or less daily events...) and do not generally venture off their planet. Also, we were never explicitly told that the Edo were accustomed to receiving interstellar guests, although it was sort of implicit.
Then again, in "Bread and Circuses" the PD was in full force even though our heroes went down with the explicit intent of contacting SS Beagle
people down there. So the level of contamination may be an issue, but not a clear-cut one.